In semiconductor manufacturing, as well as in other manufacturing processes wherein articles are chemically processed, chemicals must be cleaned from the processed article. Where the manufacturing process requires sequentially subjecting the article to several different individual chemicals, cleaning the article between chemical steps becomes especially important. For such a multi-step chemical process, cleaning is required to prevent reaction between chemicals used in processing, to prevent introduction of containments to subsequent solutions and to minimize detects formed on the article.
For example, in forming integrated circuits on a semiconductor wafer, there are several individual steps wherein the wafer is treated chemically. These steps may include developing photoresist, etching photoresist, growing oxide, and etching circuit layers. Each chemical used in a processing step (etchant, developer, etc.) may leave residue on the wafer. The residue remains until it is removed. For example, loose metal fragments (removed from the wafer during, say, an acid bath) may settle on the wafer and, so, must be cleaned off.
Typically, the wafer is passed through a series of solvent tanks to wash off the residue. At each tank, slightly less containment or residue is washed off than in the previous tank. As it is cleaned off, the contaminant remains in the cleaning solution. The final rinse is in a tank of De-Ionized (DI) water. After being rinsed in the final rinse tank, the wafer is spin dried or vapor dried.
Unfortunately, each cleaned wafer deposits contaminants in the cleaning solution in each stage. A phenomena known as cascade effect occurs when cleaning solution contamination cascades from one tank to the next, so that increasingly more cleaning stages became contaminated and fewer are left that are suitable for cleaning wafers. Eventually, because more contaminant is cleaned off in the early stages, the cleaning solution becomes so contaminated that very little is cleaned from the wafer there. So, instead of being rinsed off in the early stages, the wafers carry contaminant to subsequent stages, which also eventually become too contaminated to clean. Because of cascade effect in prior art chemical processes, contaminated solvent is replaced, continuously. Also, every time a cleaning tank was opened, solvent was contaminated with atmospheric pollutants (dust, gas etc.). So, each time wafers were transferred from one cleaning tank to the next, the solutions in both tanks become polluted from the atmosphere.
Consequently, for prior art cleaning processes, because cleaning chemically processed articles required several cleaning tanks for each step, each tank had to be capable of holding large amounts of solvent to minimize the effect of solvent contamination from each cleaned article. Each of these cleaning tanks required a large amount of manufacturing floor space. Solvent, also, had to be replaced continually because, contaminated solvent could not clean effectively. So, large volumes of solvent, requiring additional floor space, had to be stored for replacement.